It is required that a ground electrode (outer electrode) of a spark plug used in an internal combustion engine has heat resistance due to the fact that the ground electrode is subjected to high-temperature conditions. There is thus known a spark plug of the type in which a ground has a multilayer structure of a plurality of layers including a surface layer and a core formed of a higher thermal conductivity material such as copper or copper alloy inside the surface layer in order to promote heat radiation for improvement in heat resistance (see, for example, Japanese Laid-Open Patent Publication No. H11-185928). In this type of spark plug, the ground electrode is generally joined to a metal shell by resistance welding.
The heat resistance of the multilayer ground electrode increases with the amount of the core. On the other hand, the thickness of the surface layer decreases with increase in the amount of the core. The highly heat-conductive core does not largely contribute to the resistance welding strength between the ground electrode and the metal shell. Otherwise, the joint strength between the ground electrode and the metal shell decreases according to an increase in the amount of the core because the core is lower in strength than the surface layer.
Further, the surface layer spreads outwardly (as a welding burr) during the resistance welding of the ground electrode and the metal shell due to differences in melting point and strength between the surface layer and the core. A redundant part of such a spread of the surface layer is generally removed. However, the joint strength between the ground electrode and the metal shell further decrease upon removal of the part of the surface layer. The durability of the spark plug may deteriorate as the joint strength between the ground electrode and the metal shell decreases as mentioned above.